: The goal of this proposal is to test the hypothesis that monoaminergic input acts to preserve the normal threshold hierarchy among spinal motoneurons in a pool. To achieve this goal, investigations of the effects of subtype-specific monoaminergic agents on motoneuron intrinsic properties and synaptic inputs are proposed. The decerebrate cat preparation is utilized because it has both tonic activity in descending monoaminergic tracts and normal motoneuron recruitment. Reversible cold block of the spinal cord is used to eliminate endogenous monoaminergic activity. Intracellular single electrode voltage clamp (SEVC) methods provide measurements of motoneuron intrinsic properties and synaptic inputs. Our aims test the following specific hypotheses: Aim 1: that monoamines act to preserve the normal pattern of synaptic input to motoneurons. Aim 2: that monoamines generate plateau potentials and bistable behaviors in motoneurons without distorting the normal S vs. F threshold differences. Aim 3: that monoamines influence the interaction between synaptic inputs and intrinsic properties of type S and F motoneurons by allowing synaptic inputs to generate plateau potentials in dendrites even when the soma is voltage clamped. These studies have direct relevance to spinal cord injury, where loss of tonic monoaminergic input may contribute to the abnormal motor outflow seen in human patients.